This invention relates to a process for etching a thin film or foil by electrochemical machining where preferably the foil is an electrically conductive resistive alloy predominantly comprising nickel and chromium. More particularly, this invention relates to a process for manufacturing a planar electrical resistor of the type wherein a thin metallic foil is photo-etched into a pattern of isolated windows and conductive filaments to increase the current path and total resistance value of the foil. Still more particularly, this invention relates to such a process in which the foil is etched from a bonded substrate under conditions of electrochemical machining above Jacquet's plateau on the I-V characteristic curve while maintaining the anode polarization substantially constant over the foil surface to be etched at a value which, for a given flow rate and velocity of electrolyte, or a given amplitude level and frequency of the mechanically vibrated anode, is such that the rate of formation of a viscous layer and oxygen is equal to their rate of removal.
In the prior art, the manufacture of planar electrical resistors is usually accomplished by photo-etching a thin metallic foil into a pattern of isolated windows and conductive filaments in such a way that the current path through the etched foil is significantly increased and the total resistance value of the foil is multiplied accordingly. In the prior art, these resistors may be made with a foil or thin film of a suitable conductive foil, such as one predominantly comprising nickel and chromium, deposited on an isolated substrate and photo-etched with an appropriate acid or, more specifically, with an electrolytic polishing process used in conjunction with an insulating mask. The purpose of this type of etching operation is to form a grid of the conductive foil comprising the largest possible number of conductive filaments for a given surface area, since the resistance final value of the resistor is proportional to the number of filaments and the length-to-width ratio of the filaments.
In practice, the application of the aforementioned techniques is quite difficult and accompanied by a number of significant technical problems. In effect, the use of chemical etching is not entirely satisfactory because of the raggedness and irregularities of the edges of the filament in the grid. Such raggedness or irregularities in the edge contour of the filament significantly and detrimentally affect the subsequent stability of the resistor because of the electric field gradients introduced by these defects if the filaments are too closely adjacent to one another. Consequently, this process of chemical etching is limited to the manufacture of resistors having a relatively low ohmic value wherein the conductive filaments are too narrow or too close to each other. This limited value is typically of the order of 20 k.OMEGA./cm.sup.2 with a foil having a 0.5 .OMEGA./.quadrature. ohmic value.
U.S. Pat. No. 2,885,524 to Eisler and U.S. Pat. Nos. 3,405,381 and 3,517,436 to Zandman et al., describe the etching of a metallic film to a pattern which establishes a narrow conductive path of much greater total length than the dimensions of the face of a substrate. In the latter two patents, the etching is accomplished by covering the thin alloy film with a photo-sensitive masking medium. By means of photographic exposure and development, the medium is retained in contact with the surface of the film only in the desired resistor pattern and is removed from those portions where the alloy film is to be etched away. An etching process is thereafter used to remove the exposed portions of the thin alloy film.
The U.S. Pat. No. 3,174,920 to Post describes a method for producing an electrical resistance strain gauge by an electropolishing process used in conjunction with an insulating mask. In order to be electropolished, the foil to be etched is bonded to an insulated substrate and an insulated mask is used to define the areas to be etched away. The conditions of electropolishing are met when the current density at the foil surface is controlled by the formation at the surface of the foil of a viscous layer of oxides, which is a by-product of the electrolytic process. In order to reach the surface of the foil, the anions diffuse through the layer, thus producing in the current-voltage (I-V) characteristic curves a pattern where the current density is independent of the applied potential value between the anode and cathode.
The electropolishing conditions occur in the portion of the I-V characteristic curve where the current density is independent of the applied potential value, which characteristic is also called Jacquet's plateau. In this curve, however, a threshold level exists above which oxygen formation takes place at the anode surface, thus introducing discontinuities in the viscous layer at the foil surface. Then in this region above Jacquet's plateau the current density becomes progressively independent of the remains of viscous layers which stay on the surface, and increasingly dependent of the applied potential, as shown in FIG. 8.
As described by Post and others, electropolishing conditions are established by the development of a surface film on a workpiece and a viscous layer in the adjacent electrolyte. This film and layer, under these conditions, account for most of the electrical resistance near the workpiece and thus control the local current density in the workpiece.
Moreover, when an insulated mask is placed on the foil to be etched, the current density is probably smaller along the edges of the mask because it is insulated, and therefore the viscous layer is thinner in these areas. Thus, even when the anode-cathode potential is constant, the anode polarization conditions are different across the mask aperture, and a preferential attack, as described in the Post U.S. Pat. No. 3,174,920, takes place along these edges.
A feature of the Post method is to locate the adjacent regions of predominant electropolishing action in a partially or completely overlapped mode, leaving no island of material between them during etching. Under these conditions, windows in the mask and thus in the foil of only a few microns width can be obtained with an applied voltage of one to two volts and a current density of about 0.05 Amp/cm.sup.2.
It is clear, however, that if the conditions of overlap of the zones of preferential attack are not met, islands of material will remain along the center of the windows having a large width where there is no overlapping of the thinner parts of the viscous layer. Once an island is fully formed, there is no conducting path to the region of the foil adjacent thereto, and the subsequent effect on the island in the electropolishing process is practically zero. In effect, the island is electrically discontinuous with the cell. Moreover, continued attack in the foil by the electropolishing process results in undercutting the foil beneath the mask, at the very least affecting the accuracy of the resistor thus formed. In addition, the results under electropolishing conditions are not homogeneous due to the increase of the resistance of the foil as seen by the return current to the newly formed conductive filaments.
The process of electrochemical machining is known to the art, particularly in machining tough alloys such as might be used in the gas turbine industry. However, the art has not effectively applied these electrochemical machining techniques to the etching of thin foils or films and, in particular, to the manufacture of precision resistors. Moreover, the conditions under which such techniques might be used to manufacture precision resistors have not been significantly explored in the resistor art.
Accordingly, it is a broad object of the present invention to provide a method for manufacturing a precision electrical resistor having a planar configuration, high stability, and a low temperature coefficient under heavy load which avoids the edge raggedness produced by chemical etching and the inability to produce, under reliable conditions, windows of any size with electrolytic polising.
It is another object of this invention to produce a resistor of the type described by using a method of electrochemical machining characterized in that the operating conditions are above Jacquet's plateau.
It is another object of this invention to produce a resistor which has a stability on the order of a few tens of ppm/year.
It is still another object of this invention to produce a resistor having a low temperature coefficient on the order of between 0 and 15 ppm.
It is still a further object of this invention to etch a thin film or foil of resistive material under electromachining conditions of etching where, for a given electrolyte and its flow velocity, or a given amplitude level and frequency of the mechanical vibrated anode, the applied potential between the anode and cathode is such that the rate of formation of the viscous layer and oxygen at the foil surface is equal to their rate of removal.
It is still another object of this invention to provide such a method as described while maintaining the electrical resistance of the foil at a negligible value independent of the progress of the attack, preferably by using a relatively thick layer of a conductor such as copper bonded thereto, whereby the evolution of potential drop across the foil, due to the electrical current flow, is kept at a negligibly small level.
These and other objects of the invention will become apparent from the written description of the invention, taken in conjunction with the accompanying drawings.